It has been shown that with correctly designed shoring an extreme event or local failure of 540
some of its components does not necessarily lead to the progressive collapse of the entire 541
structure. Although there is a higher likelihood of local failure during construction compared 542
to the serviceability stage (i.e. column loss), the consequences of these failures can be lower in 543
terms of cost and materials (generally the loss of human lives is limited). In the cases 544
investigated with most serious consequences, it would be necessary to inspect the damage and 545
assess the safety of the structure to decide whether it can be repaired or needs demolition. In 546
terms of tolerable risk, the acceptable levels of risk given by guidelines such as IStructE [41] 547
or Annex B in EN 1991-1-7 [9] will give relatively high values of acceptable probability of 548
occurrence between 50% to 2% (corresponding to likely to rare likelihood respectively). A 549
more refined systematic risk analysis would be needed if the structure had significant potential 550
for instability during construction (i.e. Class 3 according to Harding and Carpenter [42]). 551
It can be concluded that since the consequences of an event such as the loss of the most 552
heavily loaded shore are rather small, it seems unnecessary to include explicitly such events in 553
the design phase. Nor is it necessary to consider the failure of multiple shores since this 554
probability is even smaller. It is important to note that the integrity of the building is assured 555
in such cases only assuming that both the structural design and the shoring system provided 556
are sound. It is advisable to take into account: a) the construction process when designing 557
building structures, b) accurate and validated simplified calculation methods should be used to 558
correctly estimate the loads transmitted between slabs and shores during building work [1], and 559
c) it is also important to use the correct RC construction procedures to avoid stability issues 560
during temporary support situations. Even so, there is still room for the application of 561
mitigation techniques to reduce the risk, for example by using load limiters on shores [2,10,43]. 562
These measures could contribute significantly towards reducing the high-risk of progressive 563
collapse observed in some cases during construction shown in Table 2. 564
565
6. Conclusions 566
This is the first study to focus on the structural response and damage of a building structure 567
under construction after the sudden failure of one or more shores. This is relevant in view of 568
the field evidence shown in Table 2 with many examples of hazards with intolerable high-risk 569
with relatively medium likelihood and medium/high consequences (structural failures). The 570
analysis was carried out on a real three-storey office building with RC flat-slabs designed 571
according to Eurocode and shoring designed using a state-of-the-art and validated simplified 572
calculation method providing accurate predictions of the axial loads in the shores. A dynamic 573
explicit finite element analysis was performed to evaluate different local damage scenarios: 1) 574
failure of the most heavily loaded shore, 2) failure of the joist on the most heavily loaded shore, 575
3) failure of the complete shore line on the most heavily loaded shore, and 4) the use of 576
incorrect shores. 577
In general, from all the situations analysed, the following can be concluded: 578
• When a shore fails the sharing of loads among the different slabs is critical to maintain 579
the integrity of the structure. Due to the high stiffness of the structure-shoring system 580
and high redundancy, the dynamic amplification obtained for the loads and deflections 581
were negligible (i.e. DAF = 1). This suggest that using DAF = 2 as suggested in EN 582
1991-1-7 [9] for general cases of accidental actions during construction can be rather 583
conservative and lead to unrealistic assessment of structural consequences and 584
associated risk. 585
• The results showed that scenarios 1 and 2 with least structural effects did not cause the 586
progressive collapse of neither the shoring nor the structure. In addition, slab cracking 587
was negligible and the level of consequence could be classified as “minimal” or “very 588
low” using standard risk assessment terminology. 589
• Scenarios 3 and 4 with higher structural effects resulted in the progressive collapse of 590
the shoring, although the integrity of the structure was not affected. In such cases severe 591
cracking was predicted to occur over most of the bay under study. In these situations, 592
in order to avoid undesirable serviceability performance and durability issues during 593
the operational stage, the structural safety should be evaluated in terms of damage, 594
possible repairs or demolition. 595
• Since the failure scenarios studied had little effect on the integrity of the RC structure, 596
it is not considered necessary to consider them explicitly when designing shoring 597
systems on building RC structures. However, it has been shown in this work that it is 598
very important to consider the construction process in the design of the structure and to 599
use accurate design method for calculating the shore loads during construction. 600
• The application of good design and correct building procedures will reduce the risk of 601
progressive collapse during construction which could be high and above the threshold 602
as observed in recent structural failures. There is still room for improvement in 603
understanding the behaviour of the structure/shoring system under extreme situations 604
and the application of mitigation techniques for the risk for example by using load 605
limiters on shores [2,10,43]. 606
Future works should study other specific failure scenarios of the shoring system that creates 607
a high level of dynamic displacement or develops a critical alternate load path. Failure of 608
shores, connections, joists or formwork boards might be considered for different stages of 609
construction and in different floors. 610
611 612
Acknowledgements 613
The authors would like to express their gratitude to the Spanish Ministry of Education, 614
Culture and Sport for funding received under: a) the FPU Program [FPU13/02466] and 615
complementary funding received for a stay at the University of Surrey (UK), and b) the 616
Mobility Program (Salvador de Madariaga 2017) of the Promotion of Talent and Employability 617
within the state’s Research & Innovation Program 2013-2016 [PRX17/00302]. The authors 618
would like to thank Dr. P. Olmati who developed the preliminary FE model of the structure at 619
the operational stage as part of a project sponsored by the EPSRC Impact Acceleration Account 620
held by the University of Surrey (grant ref: EP/K503939) linked with a previous project funded 621
by the EPSRC (grant ref: EP/K008153/1). 622
623
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